I was looking into designing circuits around photo-couples/opto-isolators instead of transistors, but I have found that I need an inverter of sorts. I need something that stops current flow through the outputs when current flows through the inputs. So the LED opens the circuit using the collector/emitter rather than closing it.
My understanding of photocouplers was derived from the description of a very simple four pin photocoupler I got from a computer power supply. I was very intrigued by the way it isolated inputs and outputs, so I wanted to see if I could build logical devices out of them instead of transistors. I quickly discovered that I needed something that turned the output off rather than on when the current flowed through the input.